194 PHYSIOLOGICAL CHEMISTRY. 



ically, without the bacteria, the foods would undergo complete 

 digestion and be practically all. absorbed, but before this ideal 

 condition can be reached the parasitic bacteria begin their work 

 and rob the body of part of its food. 



Acid Fermentation, just when this competition on the 

 part of the acid-producing bacteria begins is hard to say. 

 Through the upper third of the small intestine the reactions 

 are essentially those of true pancreatic digestion, and there is 

 at no time a sharp line of demarkation between this zone and 

 the following one. The point in the intestine where the acid 

 fermentations begin is a fluctuating one and must vary with 

 the time which has elapsed since the beginning of the digestion 

 as well as with the character of the food. The enzymic and 

 acid fermentation zones must besides overlap each other; that 

 is, in the central part of the tract the two kinds of changes 

 must go on simultaneously. Lactic and butyric fermentations 

 are favored by a nearly neutral medium, and this is for a time 

 secured by the slow neutralization of portions of these acids 

 formed through the alkali of the pancreatic, the intestinal and 

 the bile secretions. As the foods push farther down the neu- 

 tralizing action of the alkali becomes less and less marked, and 

 finally the characteristic acid decomposition becomes the prin- 

 cipal feature. 



In some animals this acid fermentation, to the almost com- 

 plete exclusion of putrefactive changes, is easily recognized. 

 The food of the herbivora contains an excess of pentoses, 

 starches and other carbohydrates, and these produce sugar 

 enough to furnish a large portion for intestinal fermentation. 

 The feces of these animals have not the disagreeable odors of 

 those of carnivorous animals, where the putrefactive reactions 

 are very marked and the fermentations of very minor impor- 

 tance. In animals with a mixed diet this condition can be 

 very largely changed at will by causing a variation of the food 

 given them. 



With the disappearance of the larger part of the carbohy- 

 drates through absorption and acid fermentation, the products 



